This module customises the behaviour of the
OpenGL.raw.GL.APPLE.fence to provide a more
Python-friendly API

Overview (from the spec)

This extension is provided a finer granularity of synchronizing GL command
completion than offered by standard OpenGL, which currently offers only two
mechanisms for synchronization: Flush and Finish. Since Flush merely assures
the user that the commands complete in a finite (though undetermined) amount
of time, it is, thus, of only modest utility. Finish, on the other hand,
stalls CPU execution until all pending GL commands have completed forcing
completely synchronous operation, which most often not the desired result.
This extension offers a middle ground - the ability to "finish" a subset of
the command stream, and the ability to determine whether a given command has
completed or not.

This extension introduces the concept of a "fence" to the OpenGL command
stream with SetFenceAPPLE. Once the fence is inserted into the command
stream, it can be tested for its completion with TestFenceAPPLE. Moreover,
the application may also request a partial Finish up to a particular "fence"
using the FinishFenceAPPLE command -- that is, all commands prior to the
fence will be forced to complete until control is returned to the calling
process. These new mechanisms allow for synchronization between the host
CPU and the GPU, which may be accessing the same resources (typically
memory).

Fences are created and deleted, as are other objects in OpenGL, specifically
with GenFencesAPPLE and DeleteFencesAPPLE. The former returns a list of
unused fence names and the later deletes the provided list of fence names.

In addition to being able to test or finish a fence this extension allows
testing for other types of completion, including texture objects, vertex
array objects, and draw pixels. This allows the client to use
TestObjectAPPLE or FinishObjectAPPLE with FENCE_APPLE, TEXTURE,
VERTEX_ARRAY, or DRAW_PIXELS_APPLE with the same type of results as
TestFenceAPPLE and FinishFenceAPPLE. Specifically, using the FENCE_APPLE
type is equivalent to calling TestFenceAPPLE or FinishFenceAPPLE with the
particular fence name. Using TEXTURE as the object type tests or waits for
completion of a specific texture, meaning when there are no pending
rendering commands which use that texture object. Using the VERTEX_ARRAY
type will test or wait for drawing commands using that particular vertex
array object name. Finally, DRAW_PIXELS_APPLE will wait or test for
completion of all pending DrawPixels commands. These tests and finishes
operate with the same limitations and results as test and finish fence.

One use of this extension is in conjunction with APPLE_vertex_array_range to
determine when graphics hardware has completed accessing vertex data from a
vertex array range. Once a fence has been tested TRUE or finished, all
vertex indices issued before the fence must have completed being accessed.
This ensures that the vertex data memory corresponding to the issued vertex
indices can be safely modified (assuming no other outstanding vertex indices
are issued subsequent to the fence).